Wallace discovered natural selection

While Charles Darwin sat on his revolutionary theory for 20 years, terrified of his conservative contemporaries, Wallace boldly set out to solve the great problem of the origin of species. Not afraid to announce unorthodox views, Wallace published a radically innovative theory of evolution (minus only natural selection) in an 1855 paper.

Then, while on the island of Gilolo and prompted by thoughts about the local races, Wallace hit on the idea of the struggle for existence and natural selection. He immediately wrote up his theory and posted it to Darwin on the next mail steamer.

Darwin, however, withheld the paper for perhaps two weeks before he let it become known. During this time, according to some, Darwin stole some ideas to use in his own otherwise identical theory.

Rather than having Wallace's paper published immediately on its own, which was normal practice at the time, Darwin's friends cooked up a scheme to rob the working-class Wallace of his priority and instead put their friend Darwin first. Papers by both men were read at a scientific meeting in 1858, but Darwin is remembered as the discoverer of the theory because his contribution was placed first.

This is said to be somewhat inaccurate, with the big breakthru happening this way:

In a letter written about two weeks later, Wallace mentioned that over the past four years he had found differently coloured tiger beetles on different islands which exactly matched the colour of the sand or mud where they lived. "Such facts as these puzzled me for a long time, but I have lately worked out a theory which accounts for them naturally."

Wallace already believed that new varieties of animals appeared randomly and frequently, the offspring of their parent species. If lots of varieties of various shades are constantly appearing, how does one come to perfectly match the colour of the environment? He remembered the idea of the struggle for existence. The varieties best suited to survive would be those that happened to be the right colour.

If the environment slowly changed colour, the parent species might go extinct. One of its daughter varieties could then be well adapted and replace it as the species. It could never revert back to the colour of its parent as that was now inferior. Thus through a struggle for existence, randomly generated variants would be sifted to form new species. It was a brilliant breakthrough.

Evolution is a combination of many ideas, some of which pre-dated both Darwin and Wallace. If natural selection is considered to be the big idea, then it appears that Darwin was scooped by Wallace, with Darwin rushing into print to make it appear that he had the idea all along.